1. Field of the Invention
The present invention relates to a hydrogen and power generation system comprising at least a reforming device and a combination fuel cell and ion pump, which is selectively operable in a hydrogen generation mode and an electricity generation mode when an anode thereof is supplied with a reformed gas from the reforming device. The present invention also concerns a method of shutting down such a hydrogen and power generation system.
2. Description of the Related Art
Fuel cells are a system for generating DC electric energy from an electrochemical reaction caused when anode and cathode are supplied with a fuel gas, i.e., a gas mainly containing hydrogen, and an oxygen-containing gas, i.e., a gas mainly containing oxygen.
For example, a solid polymer electrolyte fuel cell includes a power generation cell having a membrane electrode assembly sandwiched between separators. The membrane electrode assembly comprises an electrolyte membrane in the form of a polymer ion exchange membrane, and anode and cathode that are disposed on respective opposite sides of the electrolyte membrane. Usually, a predetermined number of membrane electrode assemblies and a predetermined number of separators are stacked together in a fuel cell stack for use on vehicles such as automobiles, or for use in home energy stations for meeting domestic electric power needs.
The fuel gas supplied to fuel cells normally comprises a hydrogen gas, which is generated from a raw material such as a hydrocarbon material by a reforming device. Generally, the reforming device operates to produce a reformable raw gas from a raw hydrocarbon material such as methane, LNG, or the like, and then to reform the reformable raw gas according to a water vapor reforming process, a partial oxidation reforming process, or an automatic thermal reforming process, for thereby generating a reformed gas (fuel gas).
The fuel gas generated by the reforming device needs to be converted into highly pure hydrogen gas (refined hydrogen gas), which may be compressed for storage. To this end, a combination fuel cell and ion pump disclosed in Japanese Laid-Open Patent Publication No. 2007-505472 (PCT) has been employed.
The combination fuel cell and ion pump comprises an electrochemical cell having an anode inlet for receiving a fuel, an anode outlet for discharging the fuel, a cathode inlet for receiving an oxidizer, a cathode outlet for discharging at least one of the oxidizer, refined oxygen, and refined hydrogen, a first connector, and a second connector, and a controller for providing electric charges to the first and second connectors in order to cause the electrochemical cell to act as a fuel cell for generating electricity, and for providing potentials to the first and second connectors in order to cause the electrochemical cell to act as at least one of a hydrogen pump for refining hydrogen and an oxygen pump for refining oxygen.
The combination fuel cell and ion pump is selectively operable in a hydrogen generation (hydrogen pump) mode and electricity generation (fuel cell) mode. In the hydrogen generation mode, hydrogen generated by the cathode of the combination fuel cell and ion pump is kept under a pressure, which is higher than that of the reformed gas supplied to the anode.
When the hydrogen generation mode is canceled, therefore, hydrogen gas remaining in the cathode of the combination fuel cell and ion pump needs to be depressurized or lowered in pressure. If the hydrogen gas remains under a high pressure in the cathode, then the hydrogen gas tends to diffuse into the anode and move into the reforming device.
One possible solution is to connect a depressurizing line to the cathode of the combination fuel cell and ion pump, and to treat the hydrogen gas with a flare combustor, or to vent the hydrogen gas to the atmosphere. However, since a dedicated facility is needed in order to depressurize hydrogen gas, this proposal is not economical, and the facility itself becomes large in size.